Manie's Microcruiser

[Angélique]

Originally Posted by Manie B

So here is my birdsmouth "experiment" with some offcut meranti

Hi Manie,

Compared to a round birdsmouth mast a hollow square mast would be lighter for the same strenght and stiffness. Also a square mast is a lot easier to build and it's easier to make it taper in both wall thickness and outher dimensions. To make it taper would save a lot useless weight high up there as the bending moment in the mast goes down as the hight goes up. Aerodynamical round and square are almost equal for the same dimensions. But as a square mast can be build in a bit smaller "diameter" for the same strength it would give you better aerodynamics compared to round. This aerodynamic advantage would be increased if a straight birdsmouth mast is compared to a taper square mast as the latter is easier to build taper and if because of the build difficulty taper has been skipped on the birdsmouth.

Some quotes and links for info... - - Click on in the quotes if you want to go to that thread & post.

Originally Posted by PAR

The only reason to install a square or rectangular section mast is to get some added strength through engineering. Square sticks are stronger then round in the same dimensions, meaning you can down size the square spar a bit, save weight and have the strength you need for the design's requirements.

Originally Posted by PAR

Actually a round, hollow mast, regardless of build method will be weaker then a square section of the same diameter and wall thickness. Ditto oval and rectangular sections. The corners of the square and rectangle section masts are farther from the center of gyration, making them more effective at resisting loading with the same wall thickness. Also rectangular sections aren't as bad a leading edge in regard to penetration and disturbance as it would seem. This is well proven.

Michael Storer Boat Design - Birdsmouth masts:
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While the birdsmouth mast is an elegant solution for a round mast it is not the lightest mast for a given stiffness. We have found we can actually produce much lighter masts using rectangular box sections (I always wondered why Francis Herreshoff used rectangular masts on his racing design). It is also possible for the masts to be built with much thinner walls than previously though this adds some additional labour and materials.

Michael Storer Boat Design - Goat Island Skiff - Rig and rigging details for efficient lug sails:
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The main requirement for the lug spars is that the mast be relatively stiff but the yard and boom be flexible.

With the rig set up this way the boat will have the gust response of a modern rig as the yard and boom bend and allow the sail to twist. That's why such a light boat can carry a large sail but still be easy to handle in brisk conditions.

- - click pic to enlarge

Square hollow mast, don't make the outside round like this one.

Material taken from the corners will cost you strenght and stiffness, so limit the corner rounding to the minimum which is required for wrapping with glass and/or carbon and/or kevlar, whatever is best and/or affordable, I don't know that. Have my doubts about plywood here, maybe others can say more about that.

Much more info on this topic if you dig into BoatDesign.net and on other fora as well.

Link - - Link

Good luck !
Angel

[LP]

Very informative Angelique. Points to you!

The only counterpoint I might throw in would be the way the yards would ride on a square section mast. Most likely, a padded and rounded (radiused corners) mast would not effect the yards in any way. I'm working a design with this kind of rig and the squared-section mast bothers my sense of harmony when it comes to it's interactions between the spars. A rounded top section where the yard rides could be done with no loss of strength. My fear though is a stress(compression by the yard) on the corner of the mast might cause crushing with failure of the surface finish, water ingress, etc, etc,

Perhaps others could expand on these thoughts?

[Angélique]

Originally Posted by LP

The only counterpoint I might throw in would be the way the yards would ride on a square section mast. Most likely, a padded and rounded (radiused corners) mast would not effect the yards in any way. I'm working a design with this kind of rig and the squared-section mast bothers my sense of harmony when it comes to it's interactions between the spars. A rounded top section where the yard rides could be done with no loss of strength. My fear though is a stress (compression by the yard) on the corner of the mast might cause crushing with failure of the surface finish, water ingress, etc, etc,

These are the options and their consequences for compressive contact stress between mast and yard....

1. Round mast + round yard:

   Contact area for resting and bumping is just a point so compressive stress is high.

   
   
2. Square mast + round yard: - (yard contact on a slightly radiused mast corner)

   Contact area for resting and bumping is a just a point but on a smaller radius then on a round mast so compressive stress is even higher then in situation #1.

   
   
3. Round mast + square yard:

   Contact area for resting and bumping is a line as long as the yard's width* so compressive stress is lower then in situation #1 & #2.

   
   
4. Square mast + square yard: - (yard contact on a slightly radiused mast corner)

   Contact area for resting and bumping is a line as long as the yard's width* but on a smaller radius then on a round mast so compressive stress is higher then in situation #3 but much lower then in situation #1 & #2.
   
   * width meant as outher thickness of the yard

#3 gives the lowest compressive contact stress and #4 comes near that and gives the best mast strenght and as the top end of the mast needs less strength LP's sugestion to round the mast above the lowest reef point of the yard might be a good idea.

But as weight and aerodynamics are more important at the top end of the mast then on the bottom and have a big impact on the boat's behavior and sailing capabilities I would adapt scantlings to that and taper the mast in both wall thickness and outher dimensions which means calculate the mast over whole length and not just the bottom.

To reduce compressive contact stress between mast and yard it might be sufficient to give the mast corners above the yard's lowest reef point some more radius which would get us almost at point #3 for the contact stress .

Maybe the planks can be processed on Dries' CNC router to taper them in width and in thickness and also make the scarves for the planks length joints ? That would allow for a quick mast build . .

Good luck !
Angel

P.S. - Let's not forget Fanie already spotted a nice tree for the mast, too bad that damn monkey got in front of it . . .

[Manie B]

Hi folks - thanks for the info.

It will be a round birdsmouth mast and it will be tapered.
Boom and yard will be rectangular, both tapered to the ends.

The reason why I am going this way is the popularity of round tapered masts as FREESTANDING. I have read of the good results that was achieved with square or rectangular masts that were supported with stays, but I am more comfortable with a round "stick" than a square "stick" freestanding.
The birdsmouth also has a greater contact area for glue than butt jointed.

We are working on much more data and I will publish all the figures and calculations when it all comes together.

The reason for the delay has been - what wood ??
The good news is Dries has located a local supplier that has SPRUCE at a good price. So know that we know we can get Spruce we can go ahead with final calculations.

So all round its good news and the project is back on track.
I have started experimenting with a jig to do the tapers and the results were excellent. Will post pics of that when it gets off the ground.

[Manie B]

Just a bit more info on what made me think in this direction

However, a ‘square’ section with rounded corners such as this would be unsuitable for any rig which uses mast hoops or parrel beads or a lug rig where the spar turns against the mast.

There have been several configurations used to increase the gluing areas across the width of built staves.

And several configurations which attempt to reduce wastage and at the same time produce large gluing surfaces.

Unfortunately while they will produce superb, strong masts they call for increasingly complex carpentry.

The ‘Birdsmouth’ technique developed by Noble-Masts in Bristol UK is perhaps one of the most successful of these techniques.

http://www.diy-wood-boat.com/Mast.html

There are several reasons why soft woods are the chosen type of timber used in the making of masts and spars.
The first and most obvious is that so any soft wood trees grow exceedingly tall and straight.
And because they have grown so tall and straight they have developed the ‘elasticity’ to withstand all that the elements can throw at them.
Soft woods are also more likely to be lighter in weight.
Sitka Spruce (Silver, Tideland or Menzies Spruce) has long been the top choice for mast builders.

here is a nice discussion on stiffness
http://www.woodworkforums.com/f169/gis-spars-109530/

To me a birdsmouth also looks "knitted" together, an interlocking unit that supports each other - if that makes sense
not just a couple of planks that were thrown together.
I also see the "other" attempts as justification to make the job easier, quicker, faster, and ultimately lazier.

One thing that I have learned the hard way, easy is not always better. Rather keep on slogging away, its worth it in the end.

here are some pics I got off the interenet
you can see where I am going with my "taper" jig, first cut in the "V" groove, turn around and then taper the opposite side. My side guides will be 50x50 steel tubing.

always exiting, always something on the go

[Manie B]

definately

But as weight and aerodynamics are more important at the top end of the mast then on the bottom and have a big impact on the boat's behavior and sailing capabilities I would adapt scantlings to that and taper the mast in both wall thickness and outher dimensions which means calculate the mast over whole length and not just the bottom.

and yes as well

taper the mast in both wall thickness and outher dimensions

I am working on 110 dia x 22mm thick for the bottom 2 meters and then the top 4 meters will taper down to 80 dia x 20mm thick.
so for "imperial" I am tapering a 4-1/2" mast down to 3-1/4" over the top 2/3 of the mast lenght. Total mast length is 20 ft.

Just to show my age
my country was still imperial when I started school (the dark ages)
so I only learned "metric" as a teen
the good thing is I can think in both
just goes to show my "split personality"
probably toooo much wine

[Fanie]

Actually Manie, we are in the dark ages now...

[hoytedow]

... and the beat goes on.